The basic mechanisms underlying comprehension of spoken language are unknown. We do not understand, for example, how the human brain extracts the most fundamental linguistic elements (consonants and vowels) from a complex and highly variable acoustic signal. An investigation of the cortical representation of speech sounds during categorical perception can likely shed light on this fundamental question. Categorical perception occurs when a change in a variable such as phonem ic contrast along a continuum is perceived, not as a gradual function but rather as a discrete category change. Previous research has implicated the superior temporal cortex in the processing Of speech sounds. However, how the cortex actually represents (i.e. encodes) phonemes is undetermined, mainly due to limitations of non-invasive recording techniques. The recording of neural activity di rectly from the cortical surface is a promising approach si nee it can provide both high spatial and temporal resolution. Here, I propose to examine the mechanisms of categorical speech processing by utilizing neurophysiological recordings obtained during ne urosurgical pnacedures. The principal focus of the independent ROO phase wil I be to elucidate the emergent invariant representation of phonemes in the superior tem poral gyrus that underiies categorical perception. High-density electrode anays, advanced signal processing, and direct eiectrocortical stim ulation will be utilized to unravel both local population encoding of speec h sounds in the lateral temporal cortex as well as global processing across multiple sensory and cognitiv e areas.